1. Field of the Invention
The present invention relates to a method of purifying 4,4'-diaminodiphenylmethane, which is useful as an intermediate in the preparation of polyurethane or as an additive for rubbers.
2. Description of the Related Art
4,4'-Diaminodiphenylmethane can be prepared by reacting aniline and formaldehyde. But, in addition to the desired 4,4'-diaminodiphenylmethane, many by-products such as polymeric condensates (e.g. polymethylenepolyphenylamine) and diaminodiphenylmethane isomers (e.g. 4,2'-diaminodiphenylmethane and 2,2'-diaminodiphenylmethane) are produced. Therefore, further purification of 4,4'-diaminodiphenylmethane is necessary to improve its quality.
However, using conventional rectification, cause a part of the 4,4'-diaminodiphenylmethane to be lost because of thermal deterioration, or the separation of by-products having close boiling points to that of 4,4'-diaminodiphenylmethane (such as diaminodiphenylmethane isomers) is difficult. Moreover, in an industrial scale production, rectification is not advantageous in view of energy consumption, and facilities and equipment requirements.
It has been proposed to purify 4,4'-diaminodiphenylmethane by forming an adduct of 4,4'-diaminodiphenylmethane with an alkali metal halide or cyanide, separating the adduct and decomposing the adduct to recover 4,4'-diaminodiphenylmethane (GB Patent No. 1 169 127), or by condensation reacting aniline and formaldehyde, and diluting, partially neutralizing, crystallizing and neutralizing the product (Canadian Patent No. 745,173). However, these methods are troublesome or insufficient in purification effects, or provide low productivity for the desired product per unit volume. Therefore, these methods are unsatisfactory for the industrial production of 4,4'-diaminodiphenylmethane.